I just got this to work on a 2018 Sierra using this process - worked great and total cost under $20. Key B119 from car and truck remotes and Ace cut for $10
Thanks to car keys express for info:
Adding Gm Basic Transponder Keys Must Have 2 Working Key (Manual Procedure) Up to 2017 and Up
Insert a master/working key and switch the ignition on. (I waited for security light to go out)
Insert the SECOND master/working key and switch the ignition on.
Wait for the security/immobilizer light to go out. Switch ignition off and remove key.
Quickly insert a UN-programmed key and switch the ignition on.
Wait for the security/immobilizer light to go out. Switch ignition off and remove key.
The key is now programmed.
DIC also showed that I had 3 keys programmed.
Happy Camper now!! Hope this helps someone.
